Fix welcome screen on mobile

[tordans]

Description

iD is not great to use on mobile, but it does work to check a few things from time to time. However, right now the Rapid welcome screen is not optimized for mobile and making it hard to get to the editor itself:

[bhousel]

Tagging @tannerwuster, he's going to spend some time to improve the mobile experience 🙌 This might be difficult, but maybe our dialogs can be flexboxed so that they use the space better on a small screen.

[eneerhut]

@tordans I am curious to learn more about the scenarios where you find Rapid on mobile helpful. Rapid for mobile is a topic that comes up regularly, but there are existing mobile editing solutions with varying strengths/focus areas.